Some engines may be configured to perform what may be referred to as controlled auto-ignition (CAI), whereby a mixture including air and fuel is auto-ignited during a compression stroke of the cylinder's piston without necessarily requiring a spark or a pilot injection to initiate combustion. One particular type of controlled auto-ignition known as homogeneous charge compression ignition (HCCI) includes auto-ignition of a substantially homogenous mixture of air and fuel. HCCI may be used to achieve improved engine efficiency and reduced emissions, under some conditions. However, under other conditions, it may be difficult to achieve auto-ignition or to control the timing of auto-ignition. For example, during certain higher or lower engine torque or engine speed ranges, auto-ignition may be difficult to control resulting in misfire, engine knock, or pre-ignition.
One approach to address this issue includes the use of charge temperature control to extend the operating range of auto-ignition. As one example, exhaust gases may be recirculated from the exhaust manifold to the intake manifold via an external exhaust gas recirculation (EGR) passage. These EGR gases may be used to provide charge heating, whereby the amount of EGR gases supplied to the cylinders may be adjusted to control the timing of auto-ignition. In this way, HCCI mode operation may be extended. However, the inventors of the present disclosure have recognized that this approach utilizes additional hardware including an EGR passage, EGR valves, and additional control systems, thereby increasing the cost or complexity of the engine system. As another example, a portion of the exhaust gases retained by each cylinder may be controlled by varying the timing of an exhaust valve of the cylinder. However, the inventors herein have recognized that this approach may still not provide sufficient charge heating during some conditions. For example, during lower engine load conditions, the cylinder may not retain sufficient heat to enable auto-ignition.
In order to address some of these and other issues, the inventors herein have provided a method of operating an engine including at least a first cylinder and a second cylinder. The method comprises operating the first cylinder of the engine to provide a net flow of gases from an intake manifold to an exhaust manifold for a plurality of cycles of the first cylinder and combusting a first mixture including said gases in said first cylinder by auto-ignition; and during said operation of the first cylinder, repetitively operating the second cylinder of the engine to admit gases from the intake manifold into the second cylinder and retaining the admitted gases within the second cylinder for a period before exhausting the admitted gases into the intake manifold; and adjusting an operating parameter of the second cylinder to vary a timing of said auto-ignition by the first cylinder.
In this way, gases including intake air and/or recirculated exhaust gases may be temporarily admitted to the second cylinder to enable heat transfer from the engine to the admitted gases before releasing the heated gases back into the intake manifold where they may be entrained by the first cylinder to facilitate auto-ignition. Operating parameters of the second cylinder such as spark timing, valve timing, fuel injection amount and timing, among other operating parameters of the second cylinder may be adjusted to vary the amount of heat transferred to the admitted gases and/or the amount of the gases admitted to the second cylinder and released into the intake manifold. Note that in some conditions, the second cylinder may perform combustion while trapping, heating, and releasing gases to the intake manifold, thereby also providing torque to the engine crankshaft in addition to the first cylinder.